The present invention relates generally to optical systems, and more particularly, to an all-optical up-conversion system.
The application of radio-over-fiber (ROF) systems for broadband wireless access networks has attracted much attention because it provides the high-bandwidth access services for mobile communications, wireless large-area networks, and fixed wireless access services such as local multipoint distribution systems. Fiber-optics technologies in millimeter-wave (mm-wave) wireless communication systems offer several advantages such as low-loss transmission of high-frequency signals and low electromagnetic interference. Since wavelength-division multiplexing (WDM) technology is widely used in optical transport networks from core to metro, WDM in access networks are the next logic step. In such networks, it is desirable to seamlessly integrate the WDM or WDM passive optical network (PON) transport systems with the ROF access system. An all-optical up-conversion of WDM signals for ROF provides an effective way to realize the hybrid networks.
Previous techniques to realize an all-optical up-conversion are polarization sensitive. These all-optical up-conversion methods are based on an external intensity or phase modulator and a cross-absorption modulation (XAM) in an electro-absorption modulator (EAM). These up-conversion techniques have some drawbacks like conversion loss and sensitivity to polarization of optical signals. Another all-optical up-conversion scheme is based on four-wave mixing (FWM) in the nonlinear optical fiber. With the advance of high-nonlinear dispersion-shifted fiber (HNL-DSF), highly efficient broadband FWM effect can be achieved. Because of the ultrafast nonlinear response of the fiber, it is possible to realize terahertz waveform all-optical mixing or up-conversion. Since FWM is independent of the signal bit rate and coding format, it can be used to realize up-conversion of multi-channel signals. However, the regular up-conversion scheme is polarization sensitive because only one single pump has been used.
Accordingly, there is a need for an all-optical up-conversion that is polarization insensitive and stable.